Blender Git Loki

UI: Rename operator to open Preferences window

Renames the operator from "Show Preferences" to "Open Preferences...". "Open"
is more clear than "Show" (since they could be shown in-place). "..." is
usually used in Blender to indicate that a new Window or popup will be opened.

Note that vanilla Blender doesn't actually show this name anywhere, so this
change shouldn't be visible. That may change, see D12894.